Available online at www.academicpaper.org
Academic @ Paper
ISSN 2146-9067
International Journal of Automotive
Engineering and Technologies
Vol. 4, Issue 2, pp. 68 – 81, 2015
Original Research Article
Material Selection for Dynamic Variable Geometry Turbocharger Flow
Control Application
Apostolos Pesiridis1*, Srithar Rajoo2, Kishokanna Paramasivam2, Ricardo Martinez-Botas3
and Robert Macnamara4
1School of Engineering and Design, Brunel University London 2Transport Research Alliance, Universiti Teknologi Malaysia
3Department of Mechanical Engineering, Imperial College London 4BP, Houston, TX, 77079, USA
Receive 02 April 2014, Accepted 25 June 2015
Abstract
This paper investigates nozzle material candidates for use in a turbocharger turbine technology known
as the active control turbocharger (ACT) which is a distinct technology to the Variable Geometry
Turbine (VGT) for turbochargers, but broadly based on this technology. In this concept an actuated
nozzle mechanism is oscillated to provide a continuous change of the turbine inlet area in response to
the instantaneous exhaust gas flow pulsating characteristics to provide greater extraction of exhaust gas
pulse energy. Careful materials selection is required for this application to overcome the creep, fatigue,
oxidation and high temperature challenges associated with the diesel engine exhaust conditions to which
the nozzle is exposed to. The investigation of materials suitability for this application was conducted for
steady and transient flow conditions. It was found that the nozzle vane undergo cyclical loading at a
maximum stress of 58 MPa for 109 cycles of operation at an inlet temperature of 800oC and pressure of
240 kPa. The vane experiences maximum stresses in the closed position which occurs at a vane angle
of 70o. It has been found that the implementation of ACT technology is possible using currently
available materials. A material selection process was developed to incorporate the specific application
requirements of the ACT application. A weightinG decision process was applied to analyse the
importance of various material properties to each application requirement and to the properties of
individual materials. Nimonic 90 and IN X750/751 obtained the highest overall scores from the selection
process and were shown to be capable of withstanding the creep requirements; a failure mechanism of
primary concern in the high temperature application. Nimonic 80A, although receiving a final rating 8%
lower than Nimonic 90, also showed promising potential to offer a solution, with superior corrosion
properties to both Nimonic 90 and IN X750/751.
Keywords: Variable Geometry Turbocharger, Active Control Turbocharger, superalloys, pivoting vane, CFD, FEA, transient
analysis
Nomenclature ACT Active Control Turbocharger
CFD Computational Fluid Dynamics
FEA Finite Element Analysis
VGT Variable Geometry Turbine
(Turbocharger)
𝑓 frequency
𝑁 revolution per minute
𝑛 number of strokes
𝐺 number of groups connected to the
turbine entry
𝐶 number of cylinders
𝜃 angle position (degree)
𝑇 torque
𝐼 moment of inertia
𝛼 angular acceleration
£ pounds sterling (currency)
*Corresponding author:
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1. Introduction
In the current decade, the European
automotive industry must reduce the average
emissions levels of light vehicles by 32% to
meet the 2020 target of 95 gCO2 km⁄ as
mandated by EC regulation No. 443
(European Commission, 2010).
Developments in turbocharger technology
are of particular interest to meet these
reductions, due to their ability to improve
drivability and power output while
improving fuel consumption and decreasing
emissions (Pesiridis, 2007).
Turbochargers for gasoline and diesel
engines capture energy from exhaust gases to
improve power output and contribute to
engine downsizing. Upon leaving the engine
cylinders, the hot exhaust gas is channelled
through the turbocharger turbine (Fig.1). The
turbine shaft is connected to a compressor
located upstream of the engine intake
manifold. As the exhaust gas drives the
turbine and through it the compressor, the
latter is able to draw additional air into the
engine cylinders. The higher density of air in
the cylinders allows more fuel to be
combusted and thus, more power is achieved
with each stroke.
Figure 1: Schematic of main turbocharger components
(Pesiridis, 2007)
Despite their benefits, it has been found
that current turbocharger technologies fail to
fully capture the highly dynamic energy of
the pulsating exhaust gas flow (Pesiridis and
Martinez-Botas, 2006). As such, Pesiridis
and Martinez-Botas (2007) introduced the
concept of the Active Control Turbocharger
(ACT) whereby the effective throat area of
the turbine is continually modulated by
means of a fast-response nozzle. In their
work, it was demonstrated that ACTs yield an
improvement in power between 3-7%
compared to standard VGTs. This enhanced
energy recovery capability allows the ACT to
potentially bring about greater engine weight
reduction, lower fuel consumption and
improved performance compared to
currently available turbochargers.
In the ACT, the inlet area to the turbine
rotor is adjusted in accordance with the
incoming pressure pulses. This ideally would
results in a uniformly smoother pressure field
applied to the rotor allowing for improved
power output and efficiency if the
appropriate level of instantaneous inlet area
can be provided in a timely manner. To allow
modulation of the nozzle (rack) position per
incoming exhaust pulse, the nozzle must be
able to oscillate at high frequencies which for
this study were limited to 50 Hz as
representative engine operating frequency
(speed) and against a transient flow of
exhaust gas at high temperatures between
850 – 1050°C depending on engine type
(Honeywell, 2010). Rajoo (2007) extended
the concept of ACT to include a more
conventional ring of pivoting vanes for the
nozzle around the circumference of the rotor.
The intended application of ACT for this
project was a 10 litre, Diesel engine where
Rajoo’s pivoting vane system was put to the
test (Rajoo, 2007) and around which this
study is concentrated.
The conditions of operation faced by
these vanes are extreme and, given the rapid
oscillation, unlike any other application
currently found in turbocharger or gas turbine
technology. The high-temperature, high-
frequency operation compounded with the
mechanical stress applied by the actuating
mechanism and the oxidizing environment of
the transient exhaust gas pulses impose
significant limitations on the materials that
can be used in the manufacturing of these
vanes. As such, effective materials selection
for these components is of utmost importance
and requires comprehensive understanding
of the stresses experienced by the mechanism
through the most representatively extreme
operating conditions for this application.
70
Figure 2. Pivoting Vane Mechanism (a) Isometric exploded view (b) Different positions of the vane (Arnold, 2004)
This paper focuses on identifying the
materials that can withstand the high cycle
life times, temperatures and stresses involved
in ACT operation through aerodynamic flow
simulation to obtain the mechanical forces
and resulting stresses acting on the nozzle
vane mechanism.
2. The Pivoting Vane Mechanism in Active
Control Turbocharger
The pivoting vane nozzle and sliding nozzle
mechanisms currently employed in VGT use
operate in response to engine conditions
rather than the high-frequency exhaust pulse
response required by ACT. Fig.2 shows a
simplified diagram of pivoting vane
mechanism. The vanes are actuated open and
closed by a rotating disc in order to control
the flow of exhaust across the turbine
(Arnold, 2004).
In ACT operation, the vanes rapidly oscillate
between the 40° (open position) and 70°
(closed position) vane angles to adjust the
rotor inlet area in accordance with the gas
pulse (Rajoo, 2007). The intermediate
position is described by an angle of 55°.The
frequency of oscillation representing the
engine rotational speed depends on the size
and type of the engine is provided by
Equation (1) below:
𝑓 =2𝑁𝐶𝐺
60𝑛 (1)
Where 𝑓 is the frequency of emitted exhaust
pulses and is dependent on engine speed 𝑁
(rpm), 𝑛, is the number of strokes, 𝐺, the
number of groups connected to the turbine
entry and 𝐶, the number of cylinders. Fig.3
illustrates the angular position as a function
of time based on this frequency of operation,
assuming a sinusoidal variation described by
Equation (2):
𝜃 = (𝜋 12⁄ ) sin(50(2𝜋𝑡)) (2)
Equation (2) is obtained simply by treating
the 55°position as the neutral position,
corresponding to an angle of 0 radians, and
the 70° and 40° positions as +/- π/12 radians,
respectively.
Below is outlined a method of calculating the
torque necessary to achieve the oscillation.
The procedure begins with the simple
equation that describes torque as a product of
the moment of inertia, I, and the angular
acceleration, α, as described by Equation (3):
𝑇 = 𝐼𝛼 (3)
Figure 3: Sinusoidal variation of vane angle with time
With angular acceleration calculated from
the second derivative of angular position, θ
(Equation (2)) resulting in Equation (4):
𝛼 = (100𝜋)3 12⁄ )(− sin 50(2𝜋𝑡)) (4)
Hence the final equation with torque as a
function of time is described by Equation (5).
𝑇 = 𝐼((2𝑓𝑛)3 12⁄ )(− sin 50(2𝜋𝑡)) (5)
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Figure 4: Variation of torque with time
3. Internal Combustion Engine Operating
Conditions
The raw data used for this work is based on a
10-litre diesel engine operating at 2000 rpm
which provided a worst-case scenario in
terms of emitted pulse frequency. The raw
data was obtained from a previous model of
this engine and are given in Figures 5-7. This
data was then used as boundary conditions
for the computational analysis detailed in
Section 5.
Figure 5: Raw data of pressure pulse
Figure 6: Raw data of temperature pulse
Figure 7: Raw data of mass flow pulse
4. Materials Analysis
The environment of the internal
combustion engine in which the ACT vanes
are required to operating in is characterized
by:
1. High temperature fluctuations
2. High-pressure fluctuations
3. Transient flow of corrosive
combustion products such as 𝐻2𝑂, 𝐶𝑂2
and nitrous oxides (Munz et al, 2007)
Hence, the selected material must be able
to withstand these extreme conditions
without failure. Failure refers to the
component no longer being capable of
carrying the required load and fails by
fracture or other failure mechanism. Farag
(2008) and Mazur (2005) have described
several modes of failure for components
operating under such conditions:
a) Yield failure
b) Creep failure
c) Thermal and Mechanical fatigue
d) Oxidation
As a consequence of the listed material
failure mode, the focus of this work is with
the high performance group of alloys known
as superalloys, which are capable of retaining
their physical and micro-structural properties
at elevated temperatures. The reason for the
selection of superalloys will become
apparent through a study of their
characteristics in the next section.
4.1 The Superalloys
A review of the literature shows that the
following materials are currently under
investigation for application in the various
turbocharger components:
Stainless Steels (Farag, 2008)
Superalloys (Jovanovic, 2004,
Shouren, 2008, Tetsui, 1999)
Titanium -aluminum alloys
(Jovanovic, 2004, Shouren, 2008, Tetsui,
2002, Zhang, 2001)
Silicon nitride-based ceramics
(Bocanegra-Bernal, 2010, Jovanovic,
2004)
Refractory Metals (Farag, 2008)
Figure 8 compares the oxidation and
temperature strength of several of these
72
classes of materials. It is seen that some
refractory metals are stronger at higher
temperatures but lack the ability to operate in
oxidizing conditions. Titanium-aluminium
alloys and ceramics are not displayed in
Figure 9 but they are under development due
to their advantage of having a much lower
density than the superalloys (Bocanegra-
Bernal, 2010; Shouren, 2008). However,
these materials are in the development stage
with a resultant high cost of manufacture and
limited industry experience; for these reason
they have been excluded from the present
study but could become an option in the
future. Thus, nickel-based superalloys appear
to be the most appropriate materials for
consideration for application in ACT nozzle.
Figure 8. Relative capabilities of alloy systems (Sims,
Stoloff & Hagel, 1987)
4.2 Material Properties
With the primary failure mechanisms having
been identified in the previous section, Table
1, below, summarises the principal data
associated with the failure mechanisms that
will be used as a basis for comparison in the
material selection process.
Table 1: Corresponding properties of materials
directly related to likely failure mechanisms (Dieter,
1997)
It is seen that the ultimate tensile strength
(UTS), fatigue stress, creep rate, thermal
expansion coefficient (CoTE), and
electrochemical potential are the most
important parameters to explore for the
Figure 9. Cost comparison of various materials on a
per unit mass basis. Comparison is made relative to the
cost of hot-rolled low-carbon steel sheet (Farag,
2008).
Table 2: Reasons for selection of material for ACT
vane assembly
application of interest in this work. Such
parameters have been the basis for material
selection in several reviews (Ashby, 1992,
Dieter, 1997) covering the performance of
components in similar high temperature
conditions.
Based on an iterative process to analyse each
of the material properties, several nickel-
based superalloy materials were selected as
the candidates. The materials are Inconel
600, Inconel X-750, Inconel 751, Incoloy
864, Nimonic 80A,81 and Nimonic 90. Table
2 provides the principal characteristics and
73
main areas of application of the selected
materials and therefore the reason for their
inclusion in the present study.
5. Computational Analysis
The study to characterise the levels of stress
experienced by each nozzle vane consisted of
a combination of Computational Fluid
Dynamics (CFD) and Finite Element
Analysis (FEA). The ANSYS CFD (CFX)
and ANSYS FEA packages were utilised for
the simulations. The first step involved an
aerodynamic analysis of the vane to
characterise the surrounding fluid flow and
resulting pressure and temperature profile on
the vane surface. Subsequently, FEA was
initiated to calculate the resulting stress on
the vane. Finally, a material analysis is
performed where the resulting stress is
compared to manufacturer data sheets to
assess how different materials would perform
under the prescribed conditions. Figure10
shows the fluid domain of the pivoting vanes
at angles of 40, 55 and 70 degrees
(maximum, intermediate and minimum
positions, respectively). The inwards-facing
vectors outline the inlet boundary surface,
with the outwards-facing vectors outlining
the outlet boundary surface. These domains
are repeated axisymmetrically around the Z-
axis, to account for the total of 15 vanes. The
axisymmetric assumption allows the analysis
to be performed on only one vane, rather than
creating a fluid domain incorporating all 15
vanes and thus allowing for an efficient
computational analysis to be performed.
Figure 10. Fluid Domain describing the vane at
angular position of 40°
Figure 11. Fluid Domain describing the vane at
angular position of 55°
Figure 12. Fluid Domain describing the vane at
angular position of 70°
Figure 13. Flow diagram illustrating the step by step
process used to execute the CFD and FEA simulations
5.1 Transient Study
Ideally, a transient study would need to be
carried out where a moving fluid domain
74
would be applied that replicated the vane
angle oscillation with time dependent
boundary conditions defined to match the
correct timing of the angular position.
However, the complexity of generating a
moving fluid domain sufficient to achieve
this was deemed to be of sufficiently large
complexity and of such time-intensity as to
render this undertaking unrealistic within the
project constraints. This meant that
simplifications and approximations had to be
made to best replicate actual conditions as
much as feasible.
The method employed in this study required
three fluid domains to be identified,
describing three different angular positions.
Applying a whole period of raw data from
one exhaust pulse to be used for each of these
angles would lead to unnecessarily long
simulation times and post processing. To
avoid this, it was assumed that a small
number of discrete domains was adequate to
replicate the sinusoidal trace of the pulse.
Figure 14 shows how the allocation of these
sections was split between the three domains.
Figure 14: Transient study approach
The domain of the 40° vane angle is assumed
to sufficiently describe the time bounded by
vertical green lines on Figure 14 (angle of
40° − 45°), with the domain of the 70° vane
angle assumed to sufficiently describe the
time bounded by the vertical red line (angles
of 65° − 70°). The remainder of the time
within the period of 0.02s, represented by the
blue lines which is assumed to bound by the
angle 55° vane angle (angles of 45° − 65°).
A separate transient study was run for each of
the five segments shown in Figure 14. This
required running an initial steady-state
simulation for each of the segments to
correctly initialise each transient simulation.
Table 3 outlines the important data that was
used to define each initial, steady-state
simulation. Table 3: Transient simulation initial steady state
boundary conditions
5.2 Mechanical-Structural Study
The structural analysis, although the most
important output of the process, was in fact
relatively simple to set up in comparison to
the CFD setup in CFX. With the loading
profile applicable to the blade already
established, it was simply a matter of
importing the geometry and correctly
assigning the boundary conditions
summarised in Table 4 to complete the
process. The final step was to select the
outputs required by the solver, in this case
equivalent stress, and to evaluate the results.
Table 4: Mechanical ANSYS boundary conditions
6. Results
6.1 Simulation “1” Results (𝟓𝟓°)
Simulation “1” analysed the pivoting vane
angle of 55° using the raw data
corresponding to a time segment of between
0s and 0.0025s. The area of maximum stress
illustrated in Figure 15 occurred at 40% of
chord length (from the leading edge) while
Figure 16 describes the time dependent
variations of maximum equivalent stress and
pressure difference. The stress variation at
this position is best described by the change
in pressure difference between the pressure
and suction sides at the same location. The
stress contours indicate that a maximum
value of 4.12 MPa occured for a very
concentrated area of the vane and that the
global stresses experienced are closer to a
value of 1-2 MPa. It is important to note that
the entire range of maximum stress spans
approximately 0.8 MPa. It will be seen in the
75
proceeding analyses that this is a relatively
insignificant variation in stress in comparison
to that which occurs throughout the whole
time period of 0.02s.
Figure 15: Equivalent stress contours for Simulation
‘1’
Figure 16: Maximum stress and 40% chord pressure
difference variations with time output by CFX Solver
and Post for Simulation ‘1’
6.2 Simulation “2” Results (𝟕𝟎°)
Simulation ‘2’ evaluates the raw data
occurring between 0.0025s and 0.0075s for a
70° vane angle position. Figure 17 illustrates
the geometric variation of stresses obtained
during this time period, with a maximum
value of 58.04 MPa located at approximately
60% chord and global stresses closer to 25-
30 MPa. Figure 18 indicates the variation of
maximum stress and pressure difference at
60% chord with respect to time. The
variations of the maximum stress are greater
than that seen in Simulation ‘1’and this can
be attributed to the increased angle at which
the vane is positioned relative to the flow i.e.
the closed position. With the vane positioned
to increase restriction of the flow, the
effective area seen relative to the flow
becomes greater than that seen by the 70°
position, resulting in a higher level of stress.
In addition, the much greater pressure
differentials obtained between the pressure
and suction sides, with a maximum value of
100 kPa, in comparison to 30 kPa seen in the
previous simulation.
From Figure 18, the maximum stresses
between 0.0025 and 0.0075 s can be
represented by an average stress of 45 MPa.
However, as the effect of vane angle has been
shown to exert a strong influence on the
aerodynamic and structural conditions
experienced by the vane, this decrease in
reality. The assumption that the vane is
positioned at 70° for the entire duration is
incorrect, as the vane oscillates between 65°
and 70° during this time period.This means
the calculated stresses either side of the
maximum vane angle should theoretically
decrease, minimising the average stresses
present between 0.0025-0.0075 s.
Figure 17: Equivalent stress contours for Simulation
‘2’
Figure 18: Maximum stress and 60% chord pressure
difference variations with time output by CFX Solver
and Post for Simulation ‘2’
6.3 Simulation “3” Results (𝟓𝟓°)
Simulation ‘3’ analyses the pivoting vane at
an angle of 55° using the raw data
corresponding to a time segment of 0.0075-
0.0125s. Figure 19 illustrates the global
stresses experienced by the vane at the most
extreme condition during the 0.0075-0.0125
76
s time period. The maximum stress
experienced is 4.91 MPa, with global stress
in the 1-3 MPa range. The maximum stress
variation followed the trend obtained in
Simulation ‘1’ with regards to a dependence
on the pressure difference variation at the
location of maximum stress, also described
by Figure 20 and again at a location of 40%
chord. Again, the maximum stress
experienced is relatively small in comparison
to the conditions elsewhere and should not
pose a significant mechanical integrity
concern for the vane.
Figure 19: Equivalent stress contours for Simulation
‘3’
Figure 20: Maximum stress and 40% chord pressure
difference variations with time output by CFX Solver
and Post for Simulation ‘3’
6.4 Simulation “4” Results (𝟒𝟎°)
Simulation ‘4’ refers to the analysis that used
the raw data corresponding to a time range of
0.0125-0.175s at a vane angle of 40°. The
maximum pressure, located along the
pressure side, remains relatively constant up
to a chord length of approximately 60% with
the maximum stress occurring between 2.8-
5.6 MPa. This is a deviation from the trend
exhibited by the 55° and 70° vane cases,
where the stress variation is predominately a
function of pressure difference and is a
consequence of the vane being positioned at
its ‘most open’ position and therefore being
more aligned to the absolute angle of the
incoming exhaust gases. The magnitude and
range of maximum pressure experienced in
this period of time is much greater than that
seen by 55° vane, with Figure 22 describing
a variation of approximately 140 kPa.
Figure 21: Equivalent stress contours for Simulation
‘4’
Figure 22: Maximum stress and 60% chord pressure
difference variations with time output by CFX Solver
and Post for Simulation ‘4’
Figure 23: Equivalent stress contours for Simulation
‘5’
6.5 Simulation “5” Results (𝟓𝟓°)
The final simulation reviews the results for
the 0.0175-0.02s of the pulse period at a vane
angle of 55°. Figure 23 illustrates the
maximum stress variation, which in this
instance occurs at 60% chord length, unlike
the previous two simulations involving the
77
same 55° vane position; however, it can be
seen there still exists an area of relatively
high stress at 40% of chord length.
Figure 24: Maximum stress and 60% chord pressure
difference variations with time output by CFX Solver
and Post for Simulation ‘5’
6.6 Summary of the Simulation Results
In an attempt to demonstrate the general
conditions experienced over the whole
period, Figure 25 shows both the raw data
obtained in Simulations ‘1’ to ‘5’ along with
a prediction of the actual maximum stresses
that would occur based upon the results
discussed earlier. The trend line accounts for
the three angular positions discussed and as
already discussed the effect of angular
position has been shown to have a significant
impact on the conditions experienced. The
simulation data is most accurate at time
points of 0, 0.005, 0.01, 0.015 and 0.02s,
when the vane angle and raw conditions
match exactly. The trend line was therefore
set to match the simulation data as much as
possible at these points for a more accurate
characterisation of the vane stress variation
over the pulse period.
Figure 25: Maximum stress predictions based upon
simulation outputs for entire time period of 0.02 s
7. Material Selection Process and
Recommendations
A material selection process should aim to
remove as much bias that the operator may
have towards certain materials. A useful step
in achieving this is to remove evaluation of a
specific materials’ suitability from the first
stage of the process and instead evaluate only
the importance of different material
properties. A further extension of this is to
relate these properties to the specific
application of concern, in this case the
pivoting vane ACT device. This has the
advantage of forcing the developer to justify
the decision to weigh some properties more
than others via several criteria, giving the
eventual weighting a valid backing. Another
advantage of the process is that it provides
any external user with a clear reasoning
behind weighting decisions. The quantitative
decision that is applied to each property for
each application requirement is clearly
stated, allowing an external user to gage how
much they may agree/disagree with the
rationale applied. Any selection process is
subject to differences in opinion but
increasing the number of these justifications
ultimately allows the areas where these
differences occur to be acknowledged rather
than hidden.
Table 5 displays both the application
requirements deemed to be of significant
importance and the relevant material
properties critical to the success of the
pivoting-vane component under the
conditions analysed thus far. Each of the
application requirements in Table 5 deemed
important is assigned a weight on a scale of
between 1 and 10, which reflects their
relative importance to the success of the
device. A score of 1 means the requirement
has the least significance to the success of the
device and vice versa for a score of 10. The
material properties are each evaluated against
the above application requirements and given
a score from 0-10, with 0 referring to a
property which has no impact on the success
of the requirement and 10 referring to that of
the highest importance. The product of each
requirement weight and material property
weight are then summed for each application
requirement to give a material property
weighting factor.
78
The numerical values of the material
properties of the materials under
consideration are used to evaluate their
relative strengths with respect to each other
and represented in Table 6. To remove any
element of operator opinion, the score for
each material’s property was based on a
simple calculation through which the
material with the most desirable numerical
value was assigned a maximum score of ten.
The remaining materials were then expressed
as a percentage of the optimum value and
factored to represent this percentage relative
to a score of 10 e.g. a material property that
was 80% of the best value, would receive a
score of 8. This method allows any additional
materials to be assessed by the same selection
process, simply by expressing the new
properties as a percentage of the best values.
For the qualitative properties a fair rating was
assigned a score of 4, good a score of 6 and
excellent a score of 8.
Table 5. Material criteria and application requirement weighting table
Table 6. Material selection table
Based on the total score obtained for each
material evaluated in Table 6, it is
recommended that both Nimonic 90 and IN
X750/751 be considered as strong candidates
for the material required by the pivoting vane
component. Nimonic 90 received a total score
of 17101, the highest of the six materials
considered and has superior 1000hr creep
rupture properties, shown to be vital to the
success of application. IN X750/751 received a
total score of 16986, the second highest of the
six materials and relatively identical to that
demonstrated by Nimonic 90. Both Nimonic 90
and IN X750/751 have been shown to cope
with the conditions predicted by this study,
with 1000hr creep rupture properties capable of
withstanding the maximum stresses to a
minimum safety factor of 2. By contrast it has
been demonstrated that IN 600, with a 1000hr
creep rupture strength of just 38.6 MPa, would
be insufficient at coping with the predicted
conditions by the transient study. The yield
strengths and ultimate tensile strengths for both
Nimonic 90 and IN X750/751 far exceed that
required by the application, which experiences
a maximum stress of approximately 60 MPa.
The third highest scoring material was
Nimonic 80A with a total score of 15842,
approximately 8% lower than that of Nimonic
90. A factor that may be of merit to the possible
success of Nimonic 80A is the increased levels
of corrosion resistance over the two highest
scoring alternatives. An in-house study appears
to be the most comprehensive method of
understanding the extent to which all three
materials suffer from corrosion, and whether
the improved performance of Nimonic 80A
may warrant its preferred selection despite its
79
lower total score. The 1000hr creep rupture
property of Nimonic 80A is also capable of
withstanding the predicted conditions, in this
instance to a maximum safety factor of 2.
The lower density and coefficient of
thermal expansion may ultimately contribute to
an eventual selection of Nimonic 90 as the
preferred choice. However, elements of fatigue
and cost still need to be considered before a
final decision can be made. As mentioned, due
to the relative lack of fatigue information it is
recommended this comparison be performed
in-house under conditions comparable to those
in service.
Finally, unless it can be shown there are
significant cost reductions to be made with
Nimonic 81 and Incoloy 864, their lower
respective scores of 14902 and 12310 make
them unlikely choices in comparison to the
their higher scoring alternatives. Table 7 provide a summary of the most
important material properties for some of the
materials selected to be reviewed by this
study. All properties are taken at 800℃
(unless stated otherwise) as a consequence of
the common reference temperature stated in
the manufacturer’s technical data. While for
this diesel engine application 800℃ was a
satisfactory maximum temperature target for
the simulation, with regards to fatigue
strength, data concerning cycles of the order
1 × 109 were not available in the literature
provided by the material manufacturers.
Vane oscillation cycles of 1 × 109 or above
are deemed to be close to the expected
lifetime cycle count for the vane mechanism
in a potential diesel engine application of
ACT. Cycles closer to 1 × 106 and 1 × 108
are typically the maximum referenced, so
analysing a materials’ suitability with regards
to fatigue is difficult. For the purpose of this
study fatigue properties are not considered
and as such are identified as an area of future
work for consideration.
Table 7. Selected material properties
Nominal specific costs for IN 600 and IN
X750 are £24/kg and £31/kg, respectively
(Special Metals Corporation, 2002 and
2004). Costs for Nimonic 80A, 81 and 90
could not be retrieved but can be estimated
by their composition. The cost for Nimonic
80A and 90 is estimated to similar to IN
X750 due to their similar properties
(MetalPrices, 2010).
As stated in Table 7, IN 600 has the
lowest creep rupture strength compared to
other materials. The simulation results
showed a maximum stress of 58 MPa while
IN 600 ruptures at 38.6 MPa at 750℃. The
other materials showed an acceptable
strength and Nimonic 90 has the highest
among the rest of the materials.
In addition, Table 7 shows that IN X750 has
highest coefficient of thermal expansion
among the four materials. Higher thermal
expansion is an important factor since
expansion of the vanes could give rise to vane
stickiness and therefore reliability of
operation issues.
8. Conclusions
The presented work discussed the
applicability and final selection of four
material candidates for use in a turbocharger
turbine nozzle for novel active control
turbocharger (ACT). Since the concept of
ACT involves the sinusoidal oscillation of
the nozzle at a frequency equal to that emitted
by the engine exhaust system (50Hz for this
10 litre diesel engine application at 2000
rpm) a very highly stressed component (the
nozzle vane) had to be investigated for its
mechanical integrity characteristics. The
vane design employed was taken from an in-
80
house designed and built variable geometry
turbocharger suitable for this type of engine
(Rajoo, 2007). The work involved a CFD
study followed by an FEA study to assist in
the process of selection of the best amongst
four superalloys identified as suitable
candidates given the dynamic conditions
existent at the turbocharger turbine inlet.
The difficulty and very substantial
computational effort involved in setting up a
fully transient CFD study with time-
dependent rack position variation (sinusoidal
profile of nozzle rack position of ACT) has
meant a simplification of the process through
a coarse discretisation of the pulse period
time domain.
The transient study described predicted a
maximum stress of 58.04 MPa, occurring at
a vane angle of 70⁰. In addition to the
presentation of numerical values of time
dependent maximum stresses, the work also
provides descriptions of the factors which
dictate the stress behaviour of the vane
component. Variations in differential
pressures between the pressure and suction
sides of the vane have been shown to play a
dominant role in the deformation behaviour
of the component and become substantially
more significant as the vane relative angle in
relation to the absolute flow angle increases.
It has also been shown that as a consequence,
the vane at an angle of 70⁰ commonly
experiences maximum and global stresses a
full order of magnitude higher than at angles
of 40⁰ and 55⁰. For the majority of time per
0.02 s period the vane has been shown to
exhibit stresses no greater than 10 MPa,
which is encouraging with respect to fatigue
life over the projected lifetime of the
component (at over 109 cycles). The physical
outputs of the transient study offer a robust
compromise between a relatively fast steady-
state simulation and a truly transient (flow
and vane boundary condition) reality.
Combining the multiple simulations has
provided a general description of the
conditions experienced by the pivoting vane
component across a whole time period. With
this information, the ability to select a
material has been aided through a greater
understanding of the range and variation of
the stresses which must be withstood.
Obtaining this variation and magnitude in
stress levels has allowed identification of the
peak stress levels within the time period
which can be adequate in producing
deformation by creep thus helping in the
subsequent analysis and selection of the
various superalloy material options.
The development of a material selection
process has resulted in an unbiased approach
to material evaluation with specific relevance
to the application requirements.
Improvements in density, coefficient of
thermal expansion and creep resistance have
resulted in Nimonic 90 scoring a slightly
higher score of 17101 than the previously
recommended material of IN X750/751,
which obtained a total score of 16986. The
choice between the two recommendations
will ultimately come down to both fatigue
properties and cost, factors which have not
been considered and are recommended topics
of possible future work. Improvements in
corrosion resistance of both materials may
need to be investigated through an in-house
experimental study of the capabilities of both
materials under application specific
conditions to understand the levels of
resistance required. As a consequence, the
role of Nimonic 80A in providing a potential
solution with superior corrosion resistance
properties over Nimonic 90 and IN X750/751
should not be ignored.
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